Anything related to hi-fi and sound reproduction from past, present, to future. Plus the humanitarian, environmental aspects of A / V consumer electronics manufacture and marketing.

Wednesday, December 26, 2012

Does Your Car’s High Fidelity System Have an Alternator Load Dump Filter?

Common sense dictates that your car’s electrical system
never rises above 13.8 volts DC, but is it equipped with an alternator load
dump filter nonetheless?

By: Ringo Bones

It seems like 1992 was only yesterday when “mainstream”
electrical and electronic engineers where dumbfounded on why their car audio /
car high fidelity system gave up the ghost after only eight months or so. Well,
these tenured “mainstream” engineers seems to have failed to research one of
the nasties that’s been lurking inside every automotive electrical system
called alternator load dump and how it could shorten the lives of solid-state
integrated circuit based electronics of a typical car stereo / car hi-fi system.

An alternator load dump occurs when the load to which a
generator is delivering current to is abruptly disconnected, causing a brief
but relatively high spike in output voltage from the generator. In a typical
car’s alternator-car battery system, this applies to disconnecting the car
battery while it is being charged by the alternator. Ever since automotive
technicians equipped themselves with electrical measuring equipment, it has
been found out that during alternator load dump situations, voltage
measurements can be as high as 87 to 120 volts and may take up to 400
milliseconds to go back to 13.8 volts.

Given that spec sheets of automotive solid-state integrated
circuit based power amplifiers - like the AN214 - that operate in the 12-volt
DC environment of the car can only handle a power supply voltage spike of only
48 volts for up to 500 milliseconds or so, even a “brief” alternator load dump
that more often than not exceeds this voltage is enough to shorten the lives of
solid-state integrated circuit based car hi-fi. Is there a solution to protect
one’s car hi-fi from the inherently hostile automotive electrical system?

There are commercially made / ready made alternator load
dump filter boxes available out there and due to the prevailing ignorance of a
majority of do-it-yourself electronic enthusiasts of what constitutes such
filters, they are priced as if they are made of gold by unscrupulous
manufacturers. But a reliable alternator load dump filter can easily be made by
using unused parts lying around in a typical DIY electronic enthusiast’s
electronics workbench.

A typical alternator load dump filter consists of an
inductor-capacitor or L-C filter. Usually a 100 to 300 milliHenry inductor and
a 1,000 microfarad 16 volt electrolytic capacitor whose negative terminal is
connected to the car’s negative ground. To further save costs, one can even
wind his or her own 300 milliHenry inductor using a 30-foot piece of number 20
AWG magnet wire into a ½ - inch plastic bobbin with a ½ - inch E-I silicon
steel core to increase its inductance. One can even over-engineer the
alternator load dump filter by using a 10,000 microfarad 35 volt capacitor for
the L-C filter. This type of filter can also has the benefit of further
minimize the hum to inaudibility of an AN214 IC based amplifier connected to a
12-volt full wave center tap power supply plugged into a typical home’s
220-volt 60-Hz ac current.